This invention relates to a method and a system for parallel transmission. More particularly, it relates to a method and a system for parallel transmission applied with advantage to high efficiency large capacity transmission under performing skew adjustment in parallel transmission.
Since limitations are imposed on the performance of a stand-alone computer, the recent tendency is to adopt a clustering system comprised of plural interconnected computers. In order to keep up with such tendency, the interconnection between computer nodes requires a bandwidth with a larger and larger capacity. For example, a standard for high-speed transmission on Fibre Channel is coming into widespread use.
In this standard, signals transmitted in parallel are serialized by a conversion LSI (parallel/serial conversion circuit), which are transmitted, as shown in FIG. 11a. The received serial signal is deserialized into parallel signals by a conversion LSI (serial/parallel conversion circuit).
There is a variety of limiting factors in the realization of the high speed of the serial signals. One of these limiting factors is that a cable for transmitting high-speed electrical signals attenuates high frequency signals, such that a strict limitation is imposed as to the transmission distance.
For example, in a transmission speed of an order of 1.25 Gbps, the transmission distance cannot be increased beyond approximately 40 m even if there is annexed a function of compensating frequency characteristics in the form of an equalizer. This means that only a limited system configuration is possible.
If optical signals are used, it is possible to realize a higher transmission speed and a longer transmission distance than is possible with the electrical signals. The transmission speed of up to 10 Gbps has been verified with the use of the current optical communication techniques. However, high-speed optical components are rather costly.
Thus, there is proposed a parallel transmission system of transmitting parallel signal in a computer casing directly as low-speed parallel signals, not on a sole transmission path, but on plural transmission paths as shown in FIG. 11(b).
For the parallel transmission system, it may be envisaged to use electrical signals or optical signals. The parallel transmission system may be realized not only by transmitting the low-speed signals on plural transmission paths, but also by serializing the low-speed parallel signals which are transmitted over plural transmission paths.
Meanwhile, in the parallel transmission system, there is produced skew (delay) among respective transmission paths, so that means need to be provided to synchronize the data being transmitted on the respective transmission paths. As a first method for synchronizing the data among the respective transmission paths, there is used such a method in which a frame synchronization pattern is inserted as a data header for synchronization among the transmission paths based on this fixed pattern. As the frame synchronization system in this sort of the parallel transmission paths, there is disclosed in JP Patent Kokai JP-A-63-86630 a structure in which it is unnecessary to provide a frame synchronization circuit from one transmission path to another, as shown in FIG. 12. Specifically, a frame synchronization circuit 107 is provided in a parallel transmission paths and the variable delay elements 113, 114 are controlled depending on the phase difference between the frame synchronization on this transmission path and the parallel synchronization information on another transmission path for realizing frame synchronization in the parallel transmission paths.
As a second method, there is known a method in which, in parallel transmission of divided (or split) transmission data, an identifier, a sequential number etc. is introduced as a header in each frame of signals for use in reconstructing data based on this sequential number etc. As typical of this system, there is proposed in, for example, the JP Patent Kokai JP-A-4-168841 a frame transmitting/receiving system in which an exchanging device on the transmitting side, among plural exchanging devices interconnected by plural lines, splits data of a frame, formulates plural split frames by attaching frame identifiers and split frame sequential numbers, and transmits the split frames in parallel over different lines, and in which an exchanging device on the receiving side assembles the split frames to construct a sole frame based on the frame identifier and the split frame sequential number to diminish the transmission delay time among the exchanging devices, as shown in FIG. 13.
The above-described conventional method accords a header as an overhead to every data signal, thus increasing redundancy in data or data signals. This data redundancy obstructs efficient data transfer, while placing limitations on the transmission capacity.
It is therefore a general object of the present invention to overcome the aforementioned problems.
Specifically, it is an object of the present invention to provide a method supporting high efficiency large capacity transmission by according synchronization signals for skew adjustment in the parallel transmission without recourse to data overhead.
According to an aspect of the present invention there is provided a parallel transmission method including changing the transmission wavelength on a transmitting side at a pre-set synchronization timing of data transmitted on parallel transmission paths, detecting these changes in wavelength on a receiving side, and synchronizing the data transmission among respective transmission paths of the parallel transmission paths based on the results of detection.
According to a second aspect of the present invention, there is also provided a parallel transmission method including changing the state of polarization on a transmitting side at a pre-set synchronization timing of data transmitted on parallel transmission paths, detecting these changes in polarization on a receiving side and synchronizing the data transmission among respective transmission paths of the parallel transmission paths based on the results of detection.
According to a third aspect of the present invention, there is also provided a parallel transmission apparatus including a transmitter in each of transmission paths for parallel transmission capable of outputting signals of at least two wavelengths. The transmitter transmits data signals for transmission as the transmitter switches from a first one of the wavelengths to a second one of the wavelengths at a certain synchronization timing. The parallel transmission apparatus also includes a wavelength filter provided at a pre-stage of a receiver receiving the data signals on each transmission path, for separating the first wavelength and the second wavelength from each other. Each receiver receives first data and second data separated in association with the first and second wavelengths, and causes the received first and second data to be stored in first and second storage means, respectively.
Each receiver detects the oncoming of the data signals with the maximum delay on the transmission paths. The parallel transmission apparatus also includes means for synthesizing the first and second data from the first and second storage means and for performing control to output the synthesized data simultaneously on the respective transmission paths in the timing of the data with a maximum delay.
Explanation is made of present embodiments of the present invention. In a preferred first embodiment of the present invention, the transmitting wavelength or the state of polarization is changed as shown in FIG. 1, e.g., at a partition point of data on each of parallel transmission paths, in the case of optical transmission, and the receiving side detects resulting changes in the state of transmitted light to achieve synchronization among respective transmission paths. In the case of electrical signals, synchronization of data transmission is realized such as by changing the frequency, e.g., at a partition point of data on each of parallel transmission paths.
More specifically, in a preferred first embodiment of the present invention, shown in FIG. 1, there is provided an optical transmitter in each of transmission paths for parallel transmission capable of outputting signals of at least two wavelengths (transmitters 411 to 41N), these transmitters transmitting data signals for transmission as the transmitter switches from a first one of the wavelengths to a second one of the wavelengths at a certain synchronization timing. There is also provided a wavelength filter at upstream (pre-stage) of each receiver receiving the data signals on each transmission path (wavelength filters 431 to 43N), for separating the first wavelength and the second wavelength from each other. Each receiver receives first data and second data separated in association with the first and second wavelengths, and causes the received first and second data to be stored in buffers (B11-1, B11-2), respectively, whereupon oncoming of the last coming data signal (i.e., with the maximum delay) on the transmission paths is detected. The first and second data from the buffers (B11, B12) are synthesized (or combined) and the synthesized data are simultaneously output from buffers B21 to B2N on the respective transmission paths in the timing of the last coming data (with the maximum delay).
In the present embodiment of the present invention, transmission only of a data length D, devoid of redundancy, is possible in the parallel transmission system, with the receiving side synchronizing received data. With a header length H, for example, the data transmission efficiency can be improved by a factor of 1+H/D as compared with the conventional system.
In its second embodiment, the present invention includes, as shown in FIG. 5, an optical transmitter for each transmission path which is able to output the light of different polarizations. Each of the transmitters transmits data for transmission as it switches from a first state of polarization to a second state of polarization upstream synchronization timing. There is provided on the receiving side a polarization filter for separating data of first polarization and a polarization filter for separating data of second polarization at upstream of the receivers adapted for receiving data signals of the respective transmission paths (polarization filters 531 to 53N). The first and second data separated in association with the first and second polarizations are received by the receivers and stored in first buffers (B11, B12). The oncoming of the last data signal from the transmission paths with the maximum delay is detected. The first and second data from the first buffers (B11, B12) are synthesized. The synthesized data is output simultaneously on the respective transmission paths from second buffers (B21 to B2N) in the timing of the data having the maximum delay.
In its third embodiment, the present invention provides a parallel transmission system including a multiplexer 651 for multiplexing the polarized light signals from plural optical transmitters for transmitting the multiplexed polarized light signals on transmission paths, as shown in FIG. 7. There is also provided a wavelength filter 661 for separating a first one of two polarizations separated in association with the first and second states of polarization into first and second wavelengths.
The first and second data separated in association with the first and second states of polarization, respectively, are received by the receivers and stored in the first buffers, respectively. The oncoming of the last data signal with the maximum delay is detected. The data of the second polarized light signal is directly stored in the associated first buffer. The first and second data signals from the first buffer are synthesized and the resulting synthesized data signals and data signal of the second polarization are simultaneously output on the respective transmission paths in the timing of the data signal with the maximum delay.
In its fourth embodiment, the present invention includes a multiplexer 751 on a transmitting side for multiplexing the wavelengths from plural optical transmitters and for transmitting the multiplexed wavelengths on the transmission paths, as shown in FIG. 9.